NEAR-SURFACE GEOPHYSICAL SURVEY OF THE BEATTY JUNCTION BARRIER BEACH DEPOSIT, DEATH VALLEY N. P., CALIFORNIA

We conducted a new geophysical survey on the Beatty Junction Barrier Beach Deposit (a shoreline deposit of Pleistocene pluvial Lake Manly) in Death Valley National Park. The deposit lies 2.9 km north of the Beatty Junction, at an elevation of about 30 masl (Orme and Orme, 1991, Physical Geography, v.12, p. 334-346). It is about 500 m long, 50 m wide, and 5 m high. It is oriented WSW-ENE, sloping to the east and is cut by the Beatty Junction Road. Geophysical work consisted of 6 ground-penetrating radar (GPR) lines 65-270 m long, and a seismic refraction line 100 m long. Longitudinal lines, including the seismic line, were recorded along the crest of the bar, extending 100 m W of the road cut and 250 m E of the road cut. The total difference in elevation along the portion of the bar we surveyed is only about 2 m. In addition, we surveyed 4 transverse GPR lines, oriented perpendicular to, and crossing, the bar. The transverse lines were each about 100 m long.

The seismic refraction data yielded a 3-layer model consisting of a surface layer 1-2 m thick with a velocity of 200 m/s, a second layer 4-9 m thick with a velocity of 700 m/s, and a basal unit with a velocity of 1500 m/s. The thickness of the second layer (presumably the gravel of the bar), decreases sharply from the (western) apex of the deposit to the east, suggesting that material from the parent sediment in the west was transported to the east. A GPR line shows an offlapping reflector suggesting the same thing. Shallow reflectors on the lakeward side of two of the transverse GPR lines have a distinct step-like appearance that may indicate that the crest of the bar moved lakeward with time, as described by Orme and Orme (1991). All GPR lines show a thin surface layer, about 1 m thick, that unconformably covers all reflectors in the interior of the deposit, similar to the Hanaupah Shoreline Deposit at Tule Spring, described by Ibbeken and Warnke (2002, J. of Paleolimnology, v. 23, p. 439-447). The age of the deposit is given as 153,000 +/- 12,000 years, late in marine isotope stage 6 (Orme and Orme, 1991). Since this age range overlaps with that given by Machette et al. (G.S.A. Abs. w. Progr., Vol. 35, No. 6, September 2003, p. 257-258) for the Tule Spring deposit, we consider both deposits (near) time equivalent, deposited near or during Termination II.